You don't actually need references to make such statements, these are facts that we can see daily, plus there are plenty scientifically researches that say the same thing. I am actually worried about that and many parents don't see the danger. The immune system is created in first years of life, that's why it's better to let our children play in the ground or experience "non-sterile" environments so their anti corps learn to protect them. This is the mechanism. From what I see, many children and also many grown ups are facing immune system problems because their lack of defense. This is the case of my cousin, now he is taking immublast to strengthen his system.